Geochemical prospecting



Patented Feb. 9, 1943 No Drawing. Application July 20, 1940, Serial No. $46.52!)

3 Claims. (CL23-230) The present invention is directed to a method of geochemical prospecting and geochemical well logging and particularly to that method which involves the collection of samples of soil, the recovery of a gas sample from each soil sample and the analysis of each gas sample. The methd at present finds its greatest applications in geochemical well logging, but is also applicable to areal geochemicalsurveying.

In geochemical well logging and, to a somea what lesser degree, in geochemical-surface prospecting, experience has shown that the interval of time between the collectionof the sample and its analysis has some bearing on the concentra- I tions of'significant constituents found. For a long time, particularly in geochemical well log-' ging, it was thought that the length of this period was not a matter of particular importance.- Such is actually the case where the sample of soil con tains, for example, only methane. However, where the soil sample contains heavier hydrocarbons, particularly pentane and higher, it tends to lose these higher hydrocarbons on standing.

For example, if a portion of a cutting sample containing methane and higher hydrocarbons is analyzed immediately after collection and another'portion" of the same sample is allowed to stand for-a period such as a week and analyzed.

it is frequently found that the methane contained in the cuttings does not diminish appreciably while the content of the heavier hydrocarbons f has decreased tremendously and sometimes has disappeared entirely."

It does not appear that this phenomenon was due to evaporation alone, though there may be some vapor pressure relationship between the higher hydrocarbons such that the higher hydrocarbons would disappear while methane would not be lost-to a substantial'degree. But there are possible explanations for the loss' of hydrocarbons in these samples other than evaporation. A consideration of these other possibilities would not serve anypurpose for the reason that the exact cause is not known.

These losses can be avoided if the soil samples, immediately upon collection, are reduced in temperature, as by being placed in a container of dry ice or by being placed in a refrigerator. This preservation of the sample is of extreme impor-. tance because in actualpractice, particularly in well logging, it is often dimcult to analyze the samples as soon as they 'are'obtained, because,

. for example, the well may be in the process of drilling in a remote area where it is dilflcult to move analytical equipment. This is also true where long distances separate the drilling well and the laboratory. Another advantage'oi importance exists when samples are collected at a given time for analysis later during oil-peak laboratory periods. Forexample, samples for geochemical analysis may be collected during seismic prospecting operations, and for economy's sake analyzed at convenient later times. Ob-

" viously a safe storage technique becomes of great centigrade.

value.

According to the present invention, therefore,

the samples, immediately upon their recovery, such as samples in the form of cuttings, are placed in a. container together with dry ice, 'or surroundedv by dry ice. Alternatively, a portab refrigerator may be employed.

It has been indicated that a reduction in temperature to about 0 centigrade is sufllcient for the preservation of the sample, although I prefer to lower the temperature sufllciently to freeze any moisture in the sample. Naturally the sample should be maintained at such low temperature both during transport to the laboratory and.

in storage until such time as it is analyzed.

The nature and objects of the present invention having been thus describd and illustrated. what is claimed asnew and useful and isdesired to be securedby Letters Patent is: I

1. In geochemical prospecting. in which samples of soil are collected for the purpose of recovering therefrom agas to be analyzed for signiflcant constituents the step of subjecting the soil samples immediately upon recovery to such substantially reduced temperature that such constituents as are otherwise subject to loss at ordinary temperatures are retained in the samples and holding the samplesat this reduced tem- 2; The method according to claim 1 in-which the reduced temperature is not substantially above 0 centigrade.

a. The method according to claim 1 in which.

the temperature}: reduced quickly to below 0 1 mn'aos'araa. 

